The protonmotive potential difference across the vacuo-lysosomal membrane of Hevea brasiliensis (rubber tree) and its modification by a membrane-bound adenosine triphosphatase

Publisher Website
Google Scholar
Abstract
The vacuo-lysosomes of H. brasiliensis (rubber tree) constitute a suitable model system for the study of active transport and energization at the level of the membrane of plant vacuoles. The pH gradient (.DELTA.pH) and the membrane potential (.DELTA..psi.) of vacuo-lysosomes were determined by the weak base methylamine and the lipophilic cation tetraphenylphosphonium. The values obtained depended strongly on the experimental conditions such as medium pH or K+ concentration. Under experimental conditions, i.e., pH 7.5 outside and low K+, the .DELTA.pH amounts to about 0.9 U, interior acid, and the .DELTA..psi. to -120 mV, interior negative. The .DELTA..psi. is presumably caused by the imposed K+ gradient, and the internal acidification might be a consequence of the passive proton inflow along the electric field. This explanation is sustained by the ineffectiveness of carbonyl cyanide p-trifluoromethoxyphenylhydrazone in destroying the .DELTA.pH and .DELTA..psi., whereas higher K+ concentration decreased both. Under conditions existing in vivo, the membrane potential might be significantly lower. The presence of ATP increased the acidification of the intravesicular space by 0.5 pH U to a .DELTA.pH of up to 1.4 and shifts the membrane potential at least 60 mV to a more positive value. The change of the protonmotive potential did not occur with ADP; the pH-dependence of the change was identical with the pH-dependence of a vacuo-lysosomal membrane-bound ATPase, and the effect of ATPase was prevented by the presence of the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The change of protonmotive potential difference, brought about by the ATPase, was at least 90 mV. This is evidence that a vacuo-lysosomal ATPase in plants can function as an electrogenic proton pump that transfers protons into the vacuo-lysosomal space.